Iron oxide nanoparticles have many current industrial uses, along with many emerging technical applications. They are well known as important components, for example, in magnetic storage devices, in heterogeneous and homogeneous catalysis, in sensors, and in magnetic resonance imaging (MRI) for medical diagnosis and therapeutics. The use of fuel-borne iron oxide nanoparticles to achieve a reduced combustion temperature of carbonaceous soot in a diesel particulate filter (DPF), thereby assisting in filter regeneration, is an important catalytic application of iron oxide nanoparticles. Build-up of carbonaceous material in diesel particulate filters results in increased exhaust system back pressure. The increased back pressure in turn causes reduced engine efficiency and higher fuel consumption. Reduced build-up of carbonaceous material would reduce the frequency at which filter regeneration is required, thereby reducing maintenance costs.
There are many synthetic processes for the production of metal oxides, including aqueous and hydrothermal precipitation, spray precipitation, combustion, plasma deposition and electrochemical techniques, among others. While water may be utilized as a solvent in each of these synthetic processes, aqueous reaction chemistries are particularly favored in manufacturing processes where high material through-put is desired. However, conventional aqueous processes—precipitation in particular—are often costly as they may involve multiple subsequent steps that are time and energy consuming, as well as equipment intensive.
Conventional large-scale metal oxide manufacturing processes can typically be divided into three stages: aqueous precipitation of precursor compounds, calcination to promote chemical reaction and to enhance crystallinity, followed by final particle size adjustment. In more detail, aqueous precipitation includes the initial steps of reactant dispersal, reactant delivery, particle precipitation, isolation, washing, drying, and optional impregnation with other metal ions; calcination involves heating to 400-1000° C. for several hours; followed by grinding, milling or classification to adjust the final particle size, among others steps.
One approach to reduce the number of steps in an aqueous process is to employ methods that produce a stable aqueous dispersion (suspension, sol) of the final particles directly from the initial reactants, thereby avoiding the time, cost and potential contamination inherent in the particle precipitation, isolation, and drying steps. Moreover, if the particles produced in such a direct method are sufficiently pure, wherein the chemical composition of the particles is as desired, and the particles are sufficiently crystalline, then the calcination step may also be eliminated. In addition, if the particle size and size distribution produced by such a direct method are substantially as desired, then the grinding, milling and classification steps may also be eliminated.
Direct methods to produce aqueous dispersions (suspensions) of crystalline cerium-containing oxide nanoparticles without the use of precipitation, isolation, drying, calcination, grinding, milling or classification steps, and the like, are described in commonly assigned U.S. Patent Application Publication 2010/0242342 A1, CERIUM-CONTAINING NANOPARTICLES, filed May 13, 2010, by K. J. Reed et al., wherein stable aqueous dispersions of crystalline cerium and iron containing oxide nanoparticles in a size range, for example, of 1-5 nanometers are described. However, as disclosed in comparative Example 8 and comparative Example 15 of the US 2010/0242342 reference, application of those methods to the production of iron-only oxide nanoparticles (no cerium), fails to produce stable dispersions of nanoparticles, rather unstable mixtures of particles on the order of about 200 nanometers to about 800 nanometers that form a sediment are produced.
While substantial progress has been made in eliminating manufacturing steps from the synthetic process by which stable aqueous dispersions of metal oxide nanoparticles are prepared, the use of iron oxide nanoparticles in applications such as fuel-borne combustion catalysts for the purpose of, for example, improving efficiency in the combustion chamber and for reducing the ignition (light-off) temperature for soot in the diesel particulate filter, requires that dispersions of these nanoparticles also exhibit stability in the fuel, such that the nanoparticles remain suspended and do not settle out. Thus these nanoparticles, although readily formed and suspended in a highly polar aqueous phase, must then be transferred to a substantially non-polar phase, a process known as solvent shifting. This problem is conventionally addressed by the use of particle stabilizers. However, most particle stabilizers used to prevent particle agglomeration in an aqueous environment are ill-suited to the task of stabilization in a non-polar environment. When placed in a non-polar solvent, such particles tend to immediately agglomerate and, consequently, lose some, if not all, of their desirable particulate properties. Changing stabilizers can involve a difficult displacement reaction or separate, tedious isolation and re-dispersal methods such as, for example, precipitation and subsequent re-dispersal with a new stabilizer using, for instance, a ball milling process, which can take several days and tends to produce polydisperse size frequency distributions.
One approach to simplifying the solvent shifting process employs diafiltration methods and glycol ether solvents of a polarity intermediate between that of water and those of non-polar hydrocarbons, which are used to reduce the polarity of metal oxide nanoparticle dispersions, as disclosed in commonly assigned U.S. Patent Application Publication 2010/0152077, PROCESS FOR SOLVENT SHIFTING A NANOPARTICLE DISPERSION, filed Aug. 28, 2009. Diafiltration, sometimes referred to as cross-flow microfiltration, is a tangential flow filtration method that employs a bulk solvent flow tangential to a semi-permeable membrane. However, drawbacks of these diafiltration methods include the following: relatively slow filtration rates (i.e. time consuming), substantial financial investment in equipment (e.g. pumps and microfilters), and production of a relatively large amount (e.g. several turnover volumes) of waste solvent.
Direct extraction of iron oxide nanoparticles from an aqueous dispersion into a less polar or non-polar phase containing an amphiphilic material and, optionally, a promoter agent to accelerate the rate of transfer of oxide nanoparticles from the aqueous phase into the less polar or non-polar phase, is known in the art. For example, U.S. Pat. No. 7,459,484 to Blanchard et al. discloses that the organic phase contain, in addition to an amphiphilic agent, an alcohol, more particularly a linear or branched aliphatic alcohol having 6 to 12 carbon atoms, such as 2-ethylhexanol, decanol, dodecanol and mixtures thereof, to promote particle transfer, and to improve stability of the organic colloid so formed. However, this reference fails to exemplify the use of a promoter agent, and merely describes (Example 1) a re-dispersal of amorphous particles into a solution of isostearic acid (amphiphilic agent) and ISOPAR® L (organic phase) that employed a heat treatment at 90° C. for 5½ hours. In addition, this reference teaches that it is preferable to operate the extraction at a temperature that is in the range from 60° C. to 150° C., advantageously between 80° C. and 140° C. Thus this reference discloses some of the limitations of the prior art processes, more particularly that these extractions preferably rely upon relatively high temperatures and long process times.
Thus, to date, some progress has been achieved in reducing the cost of producing and solvent shifting aqueous dispersions of iron-containing oxide nanoparticles. However, further improvements in manufacturing efficiency are desired, particularly in the case of iron oxide nanoparticle dispersions used as fuel-borne combustion catalysts or as soot combustion catalysts that require dispersion stability in either a non-polar solvent carrier or in the fuel. At the same time there is a need for stable non-polar iron oxide nanoparticle dispersions having very small average particle size and narrow size distributions, as those are expected to have the highest activity in aiding soot combustion in diesel particulate filters. It would also be very desirable to transfer oxide nanoparticles directly from the aqueous reaction mixture in which the nanoparticles are formed, to a substantially non-polar phase, at low temperatures, to reduce manufacturing hazards in dealing with combustible liquids. It is also advantageous to reduce the total process time necessary for the transfer of oxide nanoparticles from the aqueous to the final substantially non-polar phase, without the use of intermediate polarity solvents, such as glycol ethers. At the same time it is important that nanoparticle colloidal dispersions to be used as fuel additives exhibit excellent colloidal stability and good fluid flow (pour) properties at low ambient temperatures.